This invention relates to a chip-type thermistor (xe2x80x9cchip thermistorxe2x80x9d) which may be used for protection against an overcurrent.
Chip thermistors include both positive resistance-temperature coefficient (PTC) thermistors and negative resistance-temperature coefficient (NTC) thermistors. A PTC chip thermistor may be incorporated into the circuit of an electronic device so as to generate heat when an overcurrent with intensity greater than a specified level flows therethrough, thereby increasing its resistance due to its positive resistance-temperature characteristic and keeping the intensity of the current flowing into the electronic device below a certain level.
FIG. 2 shows a prior art PTC thermistor 1 comprising a thermistor element 5 having surface electrodes 3 and 4 formed on a planar PTC thermistor body 2, electrically insulating layers 6 formed on both main surfaces of the PTC thermistor element 5 so as to cover the surface electrodes 3 and 4, and outer electrodes 7 and 8 formed on both end surfaces of the PTC thermistor element 5 so as to be electrically connected to the surface electrodes 3 and 4.
The PTC thermistor 1, when used for protection against an overcurrent, for example, is required to have a low resistance value in order to reduce the power loss associated with a voltage drop. In general, however, the heat production from a PTC thermistor element increases if the resistance of its ceramic material is reduced. Since the insulating layer 6 of the PTC thermistor 1 is very thin, the heat from the PTC thermistor element 5 travels easily to the circuit board onto which the PTC thermistor 1 is mounted, having ill effects not only on the circuit board itself but also on peripheral devices near by.
It is therefore an object of this invention in view of these problems to provide an improved chip thermistor capable of reducing the rise in the temperature of the circuit board onto which it is mounted.
It is another object of this invention to provide such a chip thermistor capable of maintaining the strength of its planar thermistor element even if it is made thin in order to reduce its resistance.
A chip thermistor embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising a layered structure having a thermistor element and a substrate stacked one above the other with an electrically insulating layer in between, the thermistor element having a pair of surface electrodes on surfaces of a planar thermistor body, and a pair of outer electrodes each on a corresponding one of mutually opposite end parts of this layered structure, each of these outer electrodes being electrically connected to a corresponding one of the surface electrodes. The pair of surface electrodes is preferably each formed so as to cover a major portion of one of the main surfaces of the thermistor body and to reach the opposite main surface through one of its side surfaces, while in a mutually electrically insulated relationship. The substrate may preferably comprise another thermistor body.
With an electrically insulating plate thus attached to the surface of the thermistor element facing a circuit board onto which it is to be mounted, the heat generated by the thermistor element is more effectively inhibited from being conducted to the circuit board.